In OTL, the border of 1945 between Poland and Germany was drawn on the rivers Oder and Neisse. Now let's say that for whatever reason, maybe so that Stalin could grab even larger chunks of the Polish Kresy, or perhaps lack of support for a communistic Germany whatsoever... The border is drawn on the Elbe river instead. This would obviously put the German capital of Berlin in Poland.
Where would the new German capital be located? In OTL Bonn was chosen only as a temporary measure until Berlin could once again be made the capital. But here, Berlin is permanently under Polish control.
Frankfurt? It has historical ties as the "capital" of Germany, although it is fairly small.
Hamburg? It would be the biggest city in post-war Germany, but would be located very far off-center.
Munich? It's large and uh.. well, that's it.
Or maybe even Hannover?
What would happen to Berlin? Would it be depopulated and resurrected as a fairly small Polish town of say 100.000 inhabitants? Surely they could not let it stay a city of 4.000.000, which would put their own capital of Warsaw in shame.
Another problem that would arise in Poland would be that with even less of their eastern parts intact, Warsaw would be even more off-center and isolated than what it was in OTL. Would it be enough for the Poles to chose a new capital? Probably not, but it would still create some tension with the USSR.
This would probably strain relations between Poland and Germany even more, maybe even so far that Germany never officially accepts its new eastern border.
Well, what do you think? How big of a screw up would this be?